allison popping out of reverse

[hd2500]

about a week ago this problem occurred. I was backing in and everything was fine, but as i slowed down, i felt the truck go into neutral, then slam back into gear. so when i put it into reverse, it will act normal and you can feel it go into reverse, but about a second later, it will go into neutral. if you press the gas it will pop back into reverse and when you let off, it goes back into neutral. i checked tranny fluid and it was correct. i unplugged the tranny harness and the problem went away and it acted great. what do i need to change?!?

 

[duratothemax]

any codes? Just because the check engine light isnt on doesnt mean there are no codes...check the TCM for codes anyway.

You'll need a Tech 2 or EFILive (or a high end snapon scanner). A generic scan tool or whatever the auto parts store has wont work.

also need to know what year truck.....

Did you check the tranny fluid with the truck on a level surface, engine running, in park, and warmed up to temp??

Ben

 

[hd2500]

i have a built trans that has been great for about a year. the truck is a 2003 lb7. checked fluid levels, everything is good. checked codes at my tranny shop and there was nothing. they unplugged the tranny harness and reverse worked great, so its something to do with the electrical, but i have no idea what. thank you for your reply.

 

[hd2500]

Problem still not fixed. Tore apart valve body and everything was good. Checked wires going into tcm with voltmeter and all are positive and negative where they should be. When I unplug the nsbu switch, it holds reverse. Drove 3 hours to San bernardino to United Transmission Exchsnge which only deals with Allison's ad they plugged in their lap top and it would not read anything. So we pulled the tcm and hooked it up to their reader and it wouldn't show anything on their computer, so they said t was bad. I ordered a new tcm, had it programmed to my vin and plugged it in. It worked... For 5 minutes. Doing the same thing now, and shifting under half throttle is jerky and trying to find a gear. At WOT it will pull then redline and not shift for a second, then shift. I don't know where to go from here. Help please

 

[hd2500]

started truck this morning and let it idle for 30 seconds like usual. put it into drive and it went in, but very hard. pulling out of the driveway it felt like it went into neutral but when i got on the gas a little i realized i was stuck in 3rd. checked reverse and it wouldnt shift, just went into neutral. checked codes with my ppe and it only showed up p0700. got it towed to my tranny shop because i didnt want to delete any codes that could be there that my tool couldnt read. got there and they gave me a print out of codes. p0841, solenoid C stuck off, -0871 solenoid E stuck open, p0875 pressure switch reverse circuit, and p0777. planning on taking it to Mike L. whenever he has a appointment available if i cant get this fixed in a few days. any suggestions? thank you

 

[redrob]

Do you think your nsbu switch might be acting up?

 

[durallymax]

It sounded like an NSBU until you said P0777. Time for a rebuild.

 

[hd2500]

found the problem. just incase this happens to someone years from now and cant find the solution like i couldnt, and come across this forum, it was a torn 3c piston in the trans. thanks you to all that tried to help